This invention relates to the field of raster input scanners, and more particularly to large-format raster input scanners employing more than one CCD array.
The imaging device in a document scanner is typically one of two types: (1) page-width CCD array; or (2) miniature CCD array. While page-width CCD arrays have certain advantages over miniature CCD arrays, they are expensive to manufacture in widths such as 36", which is necessary for handling large documents such as engineering or architectural drawings. Miniature CCD arrays, on the other hand, while cheaper than page-width CCD arrays, are generally not available in the widths needed for larger documents.
For example, 5000-element miniature CCD arrays are commonly available, but 5000 elements are usually not enough for a 36"-wide scanner. The minimum resolution considered adequate for most engineering imaging applications is 200 dots per inch (dpi). Thus, for a 36"-wide scanner, a minimum of 7200 elements (36 .times. 200) is required. Allowing for some margin at either end of the scanner, an array of at least 7500 elements is preferable.
Miniature CCD arrays having 7500 elements are now becoming available, but like page-width CCD arrays, they are expensive. Also, because such arrays are long (around 2.5"), the demands on the focusing system are more stringent, which further increases the cost. In addition, a 7500-element array is limited to 200 dpi for a 36"-wide scan, which is still not enough for some engineering applications.
One solution that has been proposed is to use two or more CCD arrays, such as 5000-element miniature CCD arrays. These smaller arrays have the advantages that they are cheaper, readily available, and do not require as expensive focusing systems.
In theory, the idea is simple: locate the arrays relative to one another so that they effectively behave like a single 10,000 element (or more) array. In practice, however, this can be difficult because of the fine mechanical adjustments that are necessary to align the arrays. An alignment of .+-.1 pixel element is equivalent to 7 microns at the array surface, or (assuming a 36"-wide scan) around 0.0004" at the imaging plane.
Alternatively, the image could be pieced together using software, taking the separate image data from each CCD array and then mapping the appropriate pixels from each separate image into a single image. This approach, while it could theoretically compensate for all of the misalignment errors of the CCD arrays, has the disadvantage that it requires both a large amount of data storage and a large amount of time.